arm: riscv: qemu: Explain how to extract the generated dt
QEMU currently generates a devicetree for use with U-Boot. Explain how to
obtain it.
Also explain how to merge it to produce a devicetree with the U-Boot
features included.
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/devicetree/dt_qemu.rst b/doc/develop/devicetree/dt_qemu.rst
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+.. SPDX-License-Identifier: GPL-2.0+
+
+Devicetree in QEMU
+==================
+
+For QEMU on ARM, RISC-V and one PPC target, the devicetree is created on-the-fly
+by QEMU. It is intended for use in Linux but can be used by U-Boot also, so long
+as any nodes/properties needed by U-Boot are merged in.
+
+When `CONFIG_OF_BOARD` is enabled
+
+
+Obtaining the QEMU devicetree
+-----------------------------
+
+Where QEMU generates its own devicetree to pass to U-Boot tou can use
+`-dtb u-boot.dtb` to force QEMU to use U-Boot's in-tree version.
+
+To obtain the devicetree that qemu generates, add `-machine dumpdtb=qemu.dtb`,
+e.g.::
+
+ qemu-system-arm -machine virt -machine dumpdtb=qemu.dtb
+
+ qemu-system-aarch64 -machine virt -machine dumpdtb=qemu.dtb
+
+ qemu-system-riscv64 -machine virt -machine dumpdtb=qemu.dtb
+
+
+Merging in U-Boot nodes/properties
+----------------------------------
+
+Various U-Boot features require nodes and properties in the U-Boot devicetree
+and at present QEMU is unaware of these. To use these you must manually merge
+in the appropriate pieces.
+
+One way to do this is with dtc. This command runs dtc on each .dtb file in turn,
+to produce a text file. It drops the duplicate header on the qemu one. Then it
+joins them up and runs them through dtc to compile the output::
+
+ qemu-system-arm -machine virt -machine dumpdtb=qemu.dtb
+ cat <(dtc -I dtb qemu.dtb) <(dtc -I dtb u-boot.dtb |grep -v /dts-v1/) |dtc - -o merged.dtb
+
+You can then run qemu with the merged devicetree, e.g.::
+
+ qemu-system-arm -machine virt -nographic -bios u-boot.bin -dtb merged.dtb
+
+Note that there seems to be a bug in some versions of qemu where the output of
+dumpdtb does not quite match what is provided to U-Boot.